This invention relates generally to downhole tools energized by self-contained power supply apparatus and methods for energizing downhole tools and more particularly, but not by way of limitation, to electrical power supplies and methods incorporating an external combustion engine in powering an electrical circuit in a downhole tool.
In drilling and completing oil or gas wells, various activities need to be performed downhole. For example, downhole pressure and temperature readings need to be taken when conducting a drill stem test, and perforating guns need to be activated when perforating a casing prior to fracturing a formation. These two specific operations, as well as many others, are performed by tools which need to be energized when the tools are at their downhole locations. This energization is typically electrical energization, at least during some phase of the downhole operation.
In the past and at present, such electrical energization has been and is provided through a wireline from a source at the surface or through a self-contained battery pack located within the downhole tool. The cells in the battery pack have been chemical batteries, such as silver oxide or lithium types. The use of fuel cells (e.g., containing liquid hydrogen and oxygen) in a self-contained pack has been contemplated, but to my knowledge has never been commercially implemented.
One shortcoming of the wireline energization technique is the relative difficulty in using the wireline rather than merely using a "slick line" or retaining cable, which relative difficulty is well recognized in the industry. Additionally, because of the length of the wireline, electrical losses occur which would not occur if the power supply were wholly contained within the downhole tool. Finally, the requirement of a wireline does not lend itself to long-term tests, as the wireline truck or skid and power supply must remain at the well site. Moreover, the presence of a wireline or any cable in the well bore prohibits quickly closing off the well in an emergency unless one is willing to cut the wireline or cable and then "fish" it out at a later time.
Although battery packs overcome the two aforementioned shortcomings of wireline energization, the battery packs have relatively limited operating lives and electrical capacities whereby the operation of the downhole tool, both as to how much can be driven by a battery pack and as to how long energization can be sustained, is limited. When testing multiple parameters or conducting a long-term test, e.g., weeks or months, such limitations become particularly apparent. Although more batteries can be added to provide more capacity, such additional batteries at some point can no longer be accommodated because of the size constraints which are imposed upon all downhole tools by the size of the well bore and other known factors. Also, even though batteries can be replaced so that operations can be continued, such replacement requires a trip of the pipe string in which the battery packs are incorporated out of and back into the well bore, thereby increasing the expense of the operation. Such battery packs also have limitations as to the types of wells in which they can be readily used; this is specifically referring to deep wells (e.g., wells from two to five miles deep) because of the high pressures and temperatures which are encountered in these wells and which can detrimentally affect the chemical operations within the battery cells.
Thus, there is the need for an improved power supply for a downhole tool, which power supply is self-contained and wholly mounted within the downhole tool for obviating the necessity of a wireline, thereby achieving an advantage similar to that of the battery packs. Furthermore, however, such an improved power supply should overcome the shortcomings of the battery packs by providing for a longer operating life and by providing for more output capacity within a smaller volume than are provided by the battery packs known to me and by providing for reliable usage even in deep wells where temperatures are greater than those in which presently available batteries can operate.